1. Field of the Invention
The present invention relates to a process for producing high molecular weight polyester fibers. More particularly, the present invention relates to a process for producing polyester fibers with a high molecular weight corresponding to an intrinsic viscosity of 1.2 or more and having excellent mechanical properties, by a dry-jet wet spinning method.
2. Description of the Related Art
It is well known that polyester fibers, especially polyethylene terephthalate fibers have well-balanced physical and chemical properties and high spinning and drawing properties and thus are widely useful not only for various cloths but also for industrial uses. Particularly, polyester fibers having a high tensile strength are useful for various industrial materials, for example, tire cords.
Usual polyester fibers are produced by a melt-spinning (fiber-forming) method which is economically advantageous. Accordingly, to produce polyester fibers having a high tensile strength and useful for industrial materials, generally, a polyester polymer having a high degree of polymerization is melt-spun and the resultant undrawn filaments are drawn at a high draw ratio. However, the polyester polymers which can be formed into filaments by the usual melt-spinning method are limited to those having a melt viscosity of a certain upper limit value or less. Namely, it is difficult to produce undrawn polyester filaments substantially capable of being drawn from a polyester polymer having an intrinsic viscosity of 1.2 or more by the melt-spinning method. Therefore, it is very difficult to industrially produce polyester fibers having a high tensile strength exceeding 9 g/dtex (10 g/denier) by the melt-spinning method.
Japanese Unexamined Patent Publications (Kokai) Nos. 61-207,616, 62-30,024 and 4-73,212 disclose methods of producing polyester fibers from a polyester resin having a high degree of polymerization by dissolving the polyester resin in an organic solution and subjecting the resultant dope solution to a wet-type or dry-type filament-forming procedure.
In the methods of Japanese Unexamined Patent Publication (Kokai) Nos. 61-207,616 and 4-73,212, dichloroacetic acid and hexafluoropropanol are used as solvents for the polyester. The dichloroacetic acid exhibits a high degree of acidity and thus there are restrictions in handling of the dope solution and in types of materials usable for the spinning apparatus. Also, the hexafluoropropanol is disadvantageous in high price thereof. Therefore, the above-mentioned methods are difficult to be industrially utilized.
In the method of Japanese Unexamined Patent Publication No. 62-30,024, nitrobenzene is used as a solvent for the dope solution.
This method is, however, disadvantageous in that the dissolution of the polymer in nitrobenzene must be carried out at an undesirably high temperature of 200.degree. C. or more which unavoidably causes the degree of polymerization of the polymer to reduce during the dissolution.